Mononuclear Co(II) polypyridyl complexes: synthesis, molecular structure, DNA binding/cleavage, radical scavenging,...

Mononuclear Co(II) complexes [Co^II(L)Cl₂]; 1, [Co^II(L)(bpy)Cl]PF₆; 2, [Co^II(L)(phen)Cl]PF₆; 3 and [Co^II(L)(pic)Cl]; 4, (where L = N,N-bis(pyridin-2-ylmethyl)aniline, bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, pic = picolinic acid) were systematically synthesized and characterized by different analytical and spectroscopic methods. All the complexes were structurally identified by single-crystal X-ray diffraction analysis. Penta-coordinated complex 1 adopted a distorted trigonal bipyramidal geometry, whereas hexacoordinated complexes 2-4 have distorted octahedral geometry. The interactions of salmon sperm DNA (ss-DNA) with our synthesized complexes 1-4 were investigated by absorbance and fluorescence spectroscopy. All the complexes are very susceptible to DNA binding and exhibited binding affinities (K_b) in the order of ∼10⁴ M^-1, indicating their strong interaction with ss-DNA. The Stern-Volmer constant (K_sv) ranged from 0.46 ± 0.01 × 10⁴ to 1.08 ± 0.04 × 10⁴ M^-1, suggesting weak or moderate binding with DNA. Agarose gel electrophoresis revealed the DNA cleavage activity in vitro for 2-4, which could efficiently cleave the supercoiled plasmid DNA without any external agents; however, with the addition of H₂O₂, the cleavage property was enhanced. Live-cell imaging and other biochemical assays demonstrated the ability of Co(II) complexes 1-4 to induce significant cytotoxicity in A549 lung cancer cells with IC₅₀ values of 32.14 ± 1.3 μM, 3.14 ± 0.16 μM, 15.78 ± 0.72 μM and 18.45 ± 0.92 μM, and in MDA-MB-231 breast cancer cells with IC₅₀ values of 20.42 ± 0.92 μM, 0.41 ± 0.02 μM, 2.31 ± 0.12 μM and 9.67 ± 0.35 μM, respectively.